1. Field of the Invention
The present invention relates to an original document holding apparatus for holding original documents onto a contact glass of a copying machine, an image scanner or the like, to an image reading apparatus having the original document holding apparatus, and especially, to prevention of floating movement of the original document caused by warps or deformations of an original document pressing plate.
2. Description of Prior Art
In an image reading apparatus such as an image scanner or the like, as shown in FIG. 34(a) and FIG. 35(a), an original document S is placed on a contact glass 50, and then a reading means 51 scans to read optically information described in the original document. At this time, the original document S is held by an original document pressing plate 52 so that the original document S does not float upwardly from the contact glass 50.
A pressing-contacting sheet member 54 is attached to this original document pressing plate 52 through an elastic member 53 where resin or the like is generally molded into the original document pressing plate 52 and a sponge as a consecutive foamed body is used for the elastic member 53. A resin sheet is used for the pressing-contacting sheet member 54. As for a method of attachment of each above, it is generally used to apply adhesive tapes such as double-sided tapes onto both surfaces in their entirety of the elastic member 53 to paste the elastic member to the pressingcontacting sheet member 54, and thereafter to attach the elastic member 53 integrally formed with the pressing-contacting sheet member 54 onto the back surface side of the original document pressing plate 52.
When the resin is used for the original document pressing plate 52, however, a warp may occur according to molding conditions. At this time, when an entire surface of the sponge as an elastic member is pasted with adhesive tapes or the like to an original document pressing plate, the sponge and the pressing-contacting sheet member 54 are respectively deformed along the warp of the original document pressing plate 52 as shown in FIG. 34(b) and FIG. 35(b) (FIG. 34 shows a state of warping downwardly; FIG. 35 shows a state of warping upwardly). Molding conditions such as gate position or cooling method can control the direction of warp to some extent, but it is difficult to make the warp amount zero. Here, xe2x80x9ca state of warping downwardlyxe2x80x9d is a state where a center portion of the original document pressing plate is high and its end portions are low, and xe2x80x9ca state of warping upwardlyxe2x80x9d is a state where the end portions of the original document pressing plate are high and its center portion is low.
When these deformations occur in the pressing-contacting sheet member 54, space is made between the pressing-contacting sheet member 54 and the contact glass 50, resulting that placing uniformly the original document S in closely contact with the contact glass 50 becomes difficult. At this time, when the original document S has a curl or the like, the original document becomes afloat.
The floating movement of the original document induces problems causing image degradations such as blurred focus or blockiness caused by changes of the reflective angle of the original document. Particularly, when a contact type image sensor is used as a reading means 51, these influences become obvious.
As a means to solve these problems, there is a method, as shown in FIG. 36(a), that the shape of the elastic member 53 as a sponge is manufactured to fit the shape of the original document pressing plate 52, but this method requires special parts and manufacturing molds, thereby making costs high. There is another method, as shown in FIG. 36(b), that sponges 53a are separately placed on the back surface of the original pressing plate 52; a warping amount of the original document pressing plate 52 is measured; and, according to the amount of the original document pressing plate 52 is measured; and, according to the amount thus measured, a thickness of the sponges 53a is adjusted to keep the flatness of the pressing-contacting sheet member 54.
However, a difference in the warping amount of the original document pressing plate 52 occurs according to the molding conditions, and therefore it is needed to trace, at each time, a warping amount of the original document pressing plate 52 and to prepare sponges having different thicknesses.
Also, there is a method described in Japanese Unexamined Patent Publication (KOKAI) Heisei No. 7-43841 that another original document holding sheet is intermediately placed between the original document pressing plate and the contact glass to press closely an original document onto the contact glass, but this raises such problems as raising cost and as requiring burdensome operations such that both of the original document pressing plate and the original document holding sheet have to be opened and closed each time that an original document is replaced.
In consideration of the above problems, it is an object of the present invention to provide an image holding apparatus capable of closely contacting a pressingcontacting sheet member with an entire surface of an original document to prevent the original document from floating upwardly, and an image reading apparatus having the original document holding apparatus.
In order to accomplish the above objects, a representative structure according to the present invention of an original document holding apparatus closely contacting an original document with a contact glass surface includes an original document pressing plate for pressingly pushing the original document placed on the contact glass surface, a pressing-contacting sheet member for pressingly contacting the original document with the contact glass surface, and an elastic member or members placed between the original document pressing plate and the pressing-contacting sheet member to attach the original document pressing plate and the pressing-contacting sheet member together, where the elastic member or members are placed, within a placement range of the pressing-contacting sheet member, in a vicinity of the lowest surface in the warping shape of the original document pressing plate as well as where the pressing-contacting sheet member is attached to the original document pressing plate only through the elastic member or members.
With the above structure, the pressing-contacting sheet member can keep its flatness without receiving warp or distortion held by the original document pressing plate. Then, the entire range of the original document can be closely contacted onto the contact glass by the weight of the original document pressing plate. Since the pressing-contacting sheet member is not affected from an amount of warp of the original document pressing plate, quantitative management of the warping amount is not required.